Heart pacer

ABSTRACT

A nuclear heart pacer having a heat-to-electricity converter including a solid-state thermoelectric unit embedded in rubber which is compressed to impress hydrostatic precompression on the unit. The converter and the radioactive heat source are enclosed in a container which includes the electrical circuit components for producing and controlling the pulses; the converter and components being embedded in rubber. The portions of the rubber in the converter and in the container through which heat flows between the radioactive primary source and the hot junction and between the cold junction and the wall of the container are of thermally conducting silicone rubber. The primary radioactive source material Pu238 is encapsuled in a refractory casing of WC-222 which in turn is encapsuled in a corrosion-resistant casing of platinum rhodium, a diffusion barrier separating the WC-222 and Pt-Rh casings. The Pt-Rh casing is in a closed basket of tantalum. The tantalum protects Pt-Rh from reacting with other materials during cremation of the host, if any. The casings and basket suppress the transmission of hard X-rays generated by the alpha particles from the Pu238. The outside casing of the pacer is typically of titanium but its surface is covered by an electrically insulating coating, typically EPOXY resin, except over a relatively limited area for effective electrical grounding to the body of the host. It is contemplated that the pacer will be inserted in the host with the exposed titanium engaging a non-muscular region of the body.

United States Patent [191 Purdy et al.

[ HEART PACER [75] Inventors: David L. Purdy, Indiana; George J.

Magovern, Pittsburgh, both of Pa.; Nicholas Smyth, Bethesda, Md.

[73] Assignee: Coratomic lnc., Indiana, Pa.

[22] Filed: July 12, 1973 [21] Appl. No.: 378,636

OTHER PUBLICATIONS Myatl, Biomedical Engineering, Vol. 6, No. 5, May,1971, p. 192-196.

Primary Examiner-William E. Kamm [57] ABSTRACT A nuclear heart pacerhaving a heat-to-electricity converter including a solid-statethermoelectric unit em- [45] Feb. 18, 1975 bedded in rubber which iscompressed to impress hydrostatic precompression on the unit. Theconverter and the radioactive heat source are enclosed in a containerwhich includes the electrical circuit components for producing andcontrolling the pulses; the converter and components being embedded inrubber. The portions of the rubber in the converter and in the containerthrough which heat flows between the radioactive primary source and thehot junction and be tween the cold junction and the wall of thecontainer are of thermally conducting silicone rubber.

The primary radioactive source material Pu is encapsuled in a refractorycasing of WC222 which in turn is encapsuled in a corrosion-resistantcasing of platinum rhodium, a diffusion barrier separating the WC-222and Pt-Rh casings. The Pt-Rh casing is in a closed basket of tantalum.The tantalum protects Pt-Rh from reacting with other materials duringcremation of the host, if any. The casings and basket suppress thetransmission of hard X-rays generated by the alpha particles from the PuThe outside casing of the pacer is typically of titanium but its surfaceis covered by an electrically insulating coating, typically EPOXY resin,except over a relatively limited area for effective electrical groundingto the body of the host. It is contemplated that the pacer will beinserted in the host with the exposed titanium engaging a non-muscularregion of the body.

5 Claims, 15 Drawing Figures PATENTEB FEB] 8M5 3 866 616 sum 10F 7FIG.5.

HEART PACER REFERENCE TO RELATED DOCUMENTS This application relates toan application, Ser. No. 378,513, to David L. Purdy for ElectricalGeneraton filed July 12, 1973 and assigned to CORATOMIC INC.

B CKGRQIJN OF THE N E O This invention relates to the generation ofelectricity by thermoelectric conversion of heat from a local primarysource and has particular relationship to nuclear heart pacers orpacemakers. To the extent that this invention has other uses than inheart pacers it is understood that such uses are within the scope ofthis application.

A nuclear heart pacer includes a primary source of radioactive material,a thermoelectric converter which converts the heat from the source intoelectricity and an electrical circuit powered by the converter whichconverts the output of the thermoelectric converter into pulsations andcontrols the flow of the pulsations to the heart.

This prior-art heart pacer is encased in a conducting material such astitanium. In the use of this pacer muscle twitching has on occasionsbeen experienced. In addition, the operation of this pacer has at timesbeen irregular.

It is an object of this invention to overcome these disadvantages and toprovide a heart pacer which shall not produce muscle twitching and shalloperate with stability.

SUMMARY OF THE INVENTION This invention arises from the realization thatthe prior-art pacer, having a conducting casing throughout, makeselectrical contact with muscles of the host in areas of the casing. Thecurrent flow through the surface of the casing then actuates thesecontacted muscles to twitch. In addition, these contacted muscles,during the normal activity of the host, inject electrical pulsationsinto the prior-art pacer which confuse its op eration and render thispacer at times unreliable.

The heart pacer in accordance with this invention is encased in ametallic container which is coated with an insulator, typically EPOXYresin, except over a limited area. The pacer is inserted into the bodyof the host with the conducting area out of contact with any musculartissue of the heart.

BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of thisinvention, both as to its organization and as to its method ofoperation, together with additional objects and advantages thereof,reference is made to the following description, taken in connection withthe accompanying drawings, in which:

FIG. 1 is a view in perspective of a heart pacer in ac cordance withthis invention with parts of the outer container broken away to show theinterior;

FIG. 2 is a view in end elevation with parts sectioned of the heartpacer shown in FIG. 1;

FIG. 3 is a view in section taken along lines III-III of FIG. 2;

FIG. 4 is a view in longitudinal section of the battery of the heartpacer shown in FIGS. 1-3;

FIG. 5 is a plan view of the strap assembly which applies pressure tocompress the medium in which the thermoelectric unit is embedded;

FIG. 6 is a view in end elevation of the cylindrical bracket forpositioning the radioactive block of the capsule shown in FIG. 4 asviewed from one end;

FIG. 7 is a view in end elevation of this bracket as viewed from theopposite end;

FIG. 8 is a view in section taken :along line VIIIVIII of FIG. 7;

FIG. 9 is a view in section taken along line IX-IX of the portion ofFIG. 4 showing the thermoelectric unit;

FIG. 10 is a view in end elevation of the thermoelectric unit as viewedfrom the hot-junction end;

FIG. 11 is a view in end elevation of the thermoelectric unit as viewedfrom the cold-junction end;

FIG. 12 is a diagrammatic view showing the manner in which theindividual thermoelectric elements of the thermoelectric unit areinterconnected;

FIG. 13 is a block diagram of a typical electrical circuit of a heartpacer in accordance with this invention;

FIG. 14 is a view in perspective showing one way in which the heartpacer in accordance with this invention is installed in the host; and

FIG. 15 is a view in perspective showing another way in which the heartpacer in accordance with this invention is installed in the host.

DETAILED DESCRIPTION OF EMBODIMENT The apparatus shown in the drawingsis a heart pacer 21 (FIGS. 1-3) including a battery 23, printed circuitboards 25 and 27 (FIG. 2), a solidstate electronics package 29, astorage capacitor 31, a magnetic switch 33, a transformer 35 and anoutput assembly 37 for connecting the output of the transformer 35 tothe catheter or heart lead 39 which is placed on the heart muscle. Theboards 25 and 27 serve as a cradle for the battery 23. The battery 23and the circuit components 25-35 are embedded in a potting compound 41of a resilient material in a container 43 (FIG. 3) typically oftitanium. The potting compound 41 is typically predominately 2CN, asilicone rubber which is thermally insulating and which responds topressure like a fluid, transmitting pressure uniformly in alldirections. The 2CN is sold by Emerson-Cummings Corp. of Pittsburgh, Pa.However, a portion 47 of the compound between the cold-junction end 49of the battery 23 and the container 43 is composed of a thermallyhighlyconducting material, typically ECCOSIL 4952, a silicone rubber.ECCOSIL 4952 is sold by Microtechtronics Corp., Buffalo, NY. Thecontainer 43 is encased in a coating 50 of EPOXY resin except for awindow 51. The container 43 serves as ground for the electrical circuit23-35 and as a radio-frequency shield for the pacer and the window 51serves to connect the ground to the body of the host. The window 51projects outwardly from the remainder of the container 43 and is flushwith the EPOXY coating 51) as shown in FIG. 2. Typically, the heartpacer 21 has an overall length of 2.45 inches, a width of 1.88 inchesand a depth of 0.80 inch. Typically, the window or dimple 51 has aheighth of about 0.050 inch and is of oval shape about 1.8 inches by 1.1inches. While the outwardly extending dimple is preferred, theconducting area could also be in a recession.

The battery 23 (FIG. 4) includes a primary source 61 and a solid-statethermoelectric converter 63. The source 61 is enclosed in a highlyevacuated container 65 encompassed by a heat shield 67.

, dominately Pu Typically the source includes about 0.272 grams of Pu OThis fuel is sufficient to operate the pacer according to this inventionfor 20 years without renewal of the source. This source delivers l 17milliwatts initially and 100 milliwatts at the end of 20 years. Theblock 71 is mounted in positioning bracket 73 in inner capsule 75, ahollow sphere. The bracket 73 (FIGS. 7, 8, 9) is formed of a hollowcylinder 77 having tabs 79, spaced about 120, projecting outwardly fromone end and tabs'81, interposed between tabs 79 and spaced about 120,and projecting inwardly from both ends. The block 71 is supported withinthe bracket 73 by the tabs 81. The spacing between the tabs 81 is set toaccommodate the length of the block 71 between the tabs 81. Typically,tabs 81 at one end are first projected inwardly; the block 71 is theninserted in cylinder 77 engaging tabs 81; thereafter, tabs 81 at theother end are bent to engage and hold the block 71. The tabs 79 engagethe inner surface of sphere 75 and help to secure the bracket 73 in thesphere 75. Tabs 79 are bent inwardly from the 90 position about as shownin FIG. 4.

The inner capsule 75 is formed of hollow hemispheres 83 and 85, the rimsof the hemispheres being provided with cooperative projections 87 and 89respectively which are coextensively engaged to form the sphere 75. Thehemispheres 83 and 85 are closely dimensioned to about 0.001 inch.Typically, the sphere is composed ofa tantalum alloy, WC-222, has aninside radius of 0.125 inch and an outside radius of 0.155 inch. WC-222has the following chemical composition: W 9.6-11.2%,1-1F 22-28%, C0.008-0.0175%, Ta balance. The bracket 73 typically has a length ofabout 0.186 inch and a diameter of about 0.125 inch. The inner capsule75 is fire resistance and has high strength so that the block 77 remainslocked in the inner capsule regardless of what impacts the heart pacermay suffer and also if the host should be cremated. The tantalum alloyalso absorbs hard X-rays. The inner capsule 75 is enclosed in an outercapsule 91 including a hood 93 formed of a hemisphere from which acylinder extends; a spherical dish-shaped member 95 is sealed to the rimof the cylinder. Typically, the hermisphere and cylinder of the hood 93have an inner diameter of 0.316 inch and a thickness of about 0.010inch. The member 95 has the same thickness and is correspondinglydimensioned to sealto the hood 93. The outer capsule 91 is compoxsed ofplatinum-rhodium alloy PtlORh. The outer capsule 91 protects the source61 against corrosion and oxidation. A diffusion barrier 97, aluminumoxide, typically of about 0.001 inch thickness is disposed between theinner capsule 75 and the outer capsule 91. This barrier prevents thealloying of the inner and outer capsules 75 and 91.

During assembly the block 71 is secured in the bracket 73 with the tabs81 holding the block. The bracket 73 and block 71 are then insertedbetween the hemispheres 83, 85 and the hemispheres are joined at thesteps 87 and 89 into a sphere. The sphere is then welded in an inert-gas(argon) atmosphere. The sphere 75 is then inserted between the members93 and 95 of the outer capsule 91 and this capsule is welded at thejoint 99 between these members.

The unit 71-75-91 is disposed in a basket 101 typically of tantalum. Thebasket 101 is in the form of a hollow hemisphere terminating at its rimin a hollow cylinder. Typically, the basket 101 is about 0.005 inch inthickness. Near its outward end the basket 101 is welded to the flange103 of a flanged sleeve 105, typically of tantalum. The stem 107 of thesleeve 105 merges into the flange 103, at their inner surfaces, thetransition surface being a spherical annular surface of the same radiusas the dish-shaped member 95. The dish-shaped member is seated in thissurface. The stem 107 is welded to a disc 109, typically of tantalum.The inner surface of the disc 109 is spherical and coextensive with thespherical annular surface of the flange and of the same radius and themember 95 is seated in this surface. The disc 109 has an outwardlyprojecting rim 111.

A flanged disc 113 is brazed to the rim 111. This disc 113 is composedof the titanium alloy Ti6Al4V. A cylinder 115, also typically ofTi6Al4V, which contains the converter 63 is welded to the flange 117 ofthe disc 113. The flange 117 is trepanned to limit the flow of heat fromthe weld. The alloy Ti6Al4V has high strength and low thermalconductivity and is used for this reason.

The evacuated container 65 is defined by outer sheel 121 and thecylinder 115. These members and 121 are joined by a ring 123 typicallyof Ti6Al4V. The shell 121 is typically composed of titanium and includesa hollow hemispherical end 125 from the rim of which a cylinder extends.A hollow frusto-conical shell 127 extends from the rim of the cylinder.The cylindrical rim of the end 125 and the shell 127 are joined by aweld. A backing annulus 128, typically of titanium, is provided behindthe welded joint. The end 125 and the shell 127, throughout the majorportion of lengths, have a thickness typically of 0.010 inch. But theshell 127 flares out at its constricted end 129, to a thickness of about0.090 inch. The thickened end 129 is chamfered on the outside andflattened on the inside and is welded to the ring 123 around theflattened area. The ring 123 is internally welded to the cylinder 115. Areinforcing ring 124 internally of the cylinder 115 forms a part of awelded joint between the cylinder 115 and the ring 123. The ring istrepanned to suppress the flow of heat from the weld.

The vacuum within container 65 is maintained by a getter 131 typicallyCERRALOY 400. The getter 131 is mounted in an annular space providedbetween the sleeve 107, the flange 103, and a cylinder 133 which extendsfrom the rim of the basket 101 and is bent inwardly at teeth 135extending from its end. The getter 131 is held by a ring 137, typicallyof var-glass tubing.

The heat shield 67 has a hemispherical section 141, a cylindricalsection 143 and a conical section 145. The hemispherical section 141includes a plurality of hemispheres 147, typically of MONEL metal,concentric with sphere 75 and extending between the spherical part ofthe basket 101 and the inner surface of the end 125. The hemisphereshave dimples 149 so that their spacing is maintained. The cylindricalsection 143 includes a tape of alternate layers of MONEL metal foil 151,typically 0.125 inch wide by 0.001 inch thick, and E glass insulation153, typically, 0.125 inch wide by 0.005 inch thick. The tape is wrappedabout a center formed of the cylindrical parts of the hood 93 and thebasket 101 and the strip 133 which holds the getter 131. The cylindricalsection 143 firmly engages the basket 101 and the strip 133 and supportsthe asembly 717591, which is relatively heavy, radially and pre ventsits displacement radially. Radial displacement of the assembly 7145-91exerts a torque on the joint 115423-124 and may rupture this joint. Sothat the section 143 firmly engages the assembly 71-7591, dimples 144are pressed into the shell 125. The conical section 145 includes aplurality of hollow frusto-conical shells 161 coaxial with the cylinder115, typically composed of MONEL metal, each extending from thecylindrical shield 143 to a position above the ring 123. The shells 161are bent over perpendicularly to the cylinder 115 near the ring 123 andare engaged by an annular spacer 163, typically of Ti6Al4V.

The head 125 and the conical section 127 are initially separated pieceswith the backing ring 128 tack-welded to the hemispherical head 125. Inassembling the container 121 the ring 123 is welded to the end 129 ofthe conical section 127 of the container 65. The strip (typically 0.005inch thick) which forms the holder 133 for the getter 131 is wrappedaround the cylindrical end of the basket 101. The getter 131 and theglass tubing 137 are inserted and the teeth 135 of the strip are bent soas to hold the getter 131 and the tubing. The tape 151-153 is wrappedaround the assembly 13310193. The hemispherical shells 147 arepositioned about the hemispherical portion of the basket 101 and thehead 125 is positioned over the outermost shell 147. Th rims of theshells 147 engage the cylindrical shield 143. The frusto-conical shells161 are stacked in the frustoconical section 127 between the spacer 163and the end of the section 127. Several of the larger shells 161a are of0.005 inch thickness titanium; the other shells 161b are of 0.004 inchthick MONEL. A tube 171 of E-glass insulation is wound about each shell161 supporting the adjacent shell. The sections 125 and 127 are thenabutted with backing ring 128 extending be tween the rims of thesemembers (125, 127). The assembly is then placed in a chamber which isevacuated to a pressure of Torr and the joint between the rims of themembers 125 and 127 is welded and treated out so that the low pressureis maintained.

The thermoelectric converter 63 is disposed in cylinder 115. Theconverter 63 includes a solid-state thermoelectric unit 181 embedded orpotted in a medium 183 which responds to compression like a fluid,hydrostatically, transmitting the compression in all directions.

The thermoelectric unit 181 includes a plurality of positively andnegatively doped strips 185 and 187, typically of bismuth telluride,embedded in polymeric insulation 189 such as EPOXY. So embedded thisblock is itself stress resistant and protects the strips 185 and 187from rupture. Typically, there may be 81 of such strips 185 and 187arrayed as shown in FIG. 10. Successive negative and positive strips areinterconnected by solder strips 191 (FIGS. 11, 12) at the hot-junctionof the unit 181 and alternate pairs of positive and negative strips areinterconnected by solder strips 193 (FIGS. 10, 12) at the cold-junction.Typically, positive strip 185a and negative strip 187a areinterconnected by solder strip 191a at the hot-junction and negativestrip 187a and positive strip 185!) are interconnected by solder strip193b at the cold-junction. Diagonally positioned positive and negativeend strips 1185c (FIG. 11) and 187C are connected to output conductors201 and 203. The pairs of strips 185 and 187 of the array of strips formthermocouples connected in series between conductor 201 and conductor203.

The potting 183 includes a disc 211 (FIG. 41) ofthermally conductingresilient material, typically silicone rubber ECCOSIL 4952, interposedbetween the source 61 and the thermoelectric unit 181, a hollow cylinder213 of thermally insulating material, typically rubber, SYLGARD 184,encircling the unit 181, and a disc 215 of ECCOSIL 4952 having aninternally generally frusto-conical rearward projection. SYLGARD 184 issold by Techtronic Corporation of Buffalo, NY. The potting compound211-213-215 serves as axial support for the cylinder and prevents theassembly 71-7- 5-91-141-143 from buckling the cylinder 115.

The converter 63 is assembled in the cylinder 115. First, the cylinder211 is deposited on the trepanned disc 113. Next, the thermoelectricunit 181 is positioned centrally on the disc 211. Then the cylinder 213is deposited around the unit 181. A washer, 216, typically of titanium,is then positioned to cover the trepan groove 217 of the disc 123. Asplit ring 219, typically of Ti6A14V alloy is placed coaxially with thecylinder 115 on the washer 216. The ring 219 is resilient but has a highrestoring force. A cylindrical mass of thermally conducting material,typically ECCOSIL 4952, is then deposited in the cylindrical spacedefined by the ring 219, the washer 216 and the disc 123. Before thismass is cured a plug 231 is positioned at the outer edge ofthe mass.

The plug 231 is typically composed of electrolytic grade copper and hasa head 237 split or slotted at the center and a body 239 whichterminates in a generally frusto-conical portion. The body 239 has acentral opening which Communicates with an opening 241 in the head. Thecentral opening has a ceramic bushing 243.

The conductors 201 and 203 are brought out centrally before thecylindrical mass is deposited within the inner ring 219, the washer 216and the disc 123. After the mass is deposited, but before it cures, theconductors 201 and 203 are strung through the ceramic bushing 243 of theplug 231 and brought out through the slot between the parts of the head237. The plug 231 is then set at the outer end of the mass of thermallyconducting material, while being maintained coaxial with the cylinder115. As the mass is cured the ring 219 prevents the mass from expandingradially. Once the mass is cured the plug 231 is pressed into the massbuilding up hydrostatic pressure within the potting material 183. Thesplit ring 219 confines the mass radially but the gaps in this springopen to a predetermined spacing which measures the compression of themass. The force exerted on the mass of ECCOSIL 4952 may be as high aspounds; however, where the thermoelectric unit 181 is stress-resistant,the force may be as low as 15 pounds. When the gap has the desiredspacing the mass is secured by straps 233 of spring 235. The spring 235(FIG. 5) is composed of sheet titanium alloy, typically about 0.005 inchin thickness. This spring has an annular center 251 from which thestraps 233 extend radially uniformly spaced around its periphery. lnsecuring the plug 231, the center 251 is spot welded to the slopingshoulder 253 of the head 237 and the straps 233 are bent around the headand spot welded to the thickened portion 129 of the shell 127.

Initially the container 43 (FIG. 3 which is composed of commerciallypure titanium is in two parts. One part has an opening 261 for theoutput conductor 263 from the transformer 35. Initially, the battery 23,the printed circuit boards 25 and 27, block 29, containing the circuitcomponents (integrated circuit and separate transistors not shown)potted in EPOXY resin, the storage capacitor 31, the magnetic switch 33and the transformer 35 are assembled and connected outside of thecontainer 43.

The output conductor 263 is also initially connected into a feed-throughassembly 264 (FIG. 3). This assembly includes a ferrite radio-frequencyfilter 265 which suppresses electromagnetic disturbances and whichencircles the conductor 263. The assembly also inlcudes a ceramicinsulating sleeve 267 throgh which the conductor 263 is sealedgas-tight. The sleeve 267 is sealed into a flanged sleeve 269, typicallyof titanium. The ferrite trap 265 is secured to the inner side of theflanged sleeve 269 by a spring washer 271. The conductor 263 isconnected near its external end to a flexible connector 272.

Once the components are interconnected the battery 23 and the parts25-35 are mounted in one of the parts of the container 43 with thebattery cradled between the boards 25 and 27. The boards 25 and 27 arecontoured to rest on the inner surface of the container 43. The groundconductors (FIG. 13) are connected to the container 43. The assembly 264is disposed adjacent the opening 261 with the flanges 273 of sleeve 269appropriately positioned adjacent the hole 261. The two parts of thecontainer 43 are then welded to form the container. The ECCOSIL 4952mass 47 between the cold-junction end 49 of the battery and thecontainer 43 is injected with a syringe inserted through opening 261.The 2CN potting rubber is then injected through opening 261 andencompasses the parts 23 through 35. The container 43 and its contentare then placed in a chamber (not shown) which is evacuated and filledwith an inert gas (argon) at about one atmospheric pressure. The inertgas permeates the converter 63 through the opening 231 and the bushing243. Within the chamber, in the inert-gas atmosphere, the flange 273 iswelded gas-tight to the rim of the hold 261 sealing the hole. Theconnector 272 is now connected to a terminal block 275.

The terminal block 275 is in the form ofa rectangular parallelapipedhaving a cylindrical opening through which the inner end 277 of thecatheter 39 passes. Laterally a set screw 279 (FIG. 2) is provided inthe block 275. Over the head of the set screw 279 there is a silasticplug 281.

The catheter 39 is inserted in heart pacer 21 by the doctor who installsit in the hosts heart. During the construction of the pacer 21 thecatheter 39 is replaced by a pin (not shown) of the diameter of thecatheter 39. This pin is encircled by a suture boot 283. The pacer asnow assembled is mounted in a mold (not shown) with the dimple 51 whichforms the window facing downwardly and masked. The EPOXY resin 50 isthen molded about the titanium casing except at the dimple 51 and theEPOXY excapsulation 285 for the conductor 263, the connector 272, theblock 275, the bottom 283 and the pin (not shown) is formed. The pin isthen removed.

After installation in the heart ofthe host, the catheter 39 is insertedin the opening formed by the pin and the end 277 is secured by the setscrew 279 in the terminal block. The head of the set screw is closed bythe plug 281 and the plug 281 can be sealed by silastic insulation.

The electrical circuit (FIG. 13) used in the practice of this inventionis of the solid-state type and includes, in addition to the magneticswitch 33, a DC to DC converter 301, and amplifier 303, a monostable305, a noise-rate turn-on 307, a multivibrator reset 309, amultivibrator 311, and an output 313.

The output of the battery 23 is about 0.6 volt open circuit; the DC-DCconverter 301 derives about 5.4 volts from this output for operating theremainder of the circuit.

The amplifier 303 amplifies any input signal impressed on its inputterminal 305 which is greater than 1.5 millivolts to 2.0 millivolts.This stage amplifies R- waves from the heart, pacer pulses and any noisewhich might be present at the input 315. The gain of this amplifier 303falls off rapidly once the frequency of the incoming noise is out of theampliflers band pass.

The monostable 305 performs two different functions. First, when it istriggered by a signal from the amplifier 303, it puts out typically a0.275 second duration square pulse; and the monostable 305 cannot betriggered on again until it times out. This pulsing interval serves asthe pacers refractory period. The pacers mode cannot be affected duringthis time. The amplifier 303 ramains sensitive, but the multivibrator311 cannot be reset until the monstable 305 completes its pulse of 0.275second. The amplifier 301 remains operative to drive the noise rateturn-on 307. Second, the leading edge of the monostable pulse resets themultivibrator 311.

The multivibrator 311 controls the rate and width of the output pulsesof the pacer 21. The multivibrator 311 can operate in two differentmodes. One mode is the normal rate which is 702 beats per minute. Theother is the noise rate which is beats per minute. In both modes thepulse duration remains at I.Ii'.0l milliseconds.

The output 313 is the stage which produces the nega-v tive pulse whichactually paces the heart.

The multivibrator reset 309 stops the multivibrator 311 from putting outa pulse (heart pacing pulse). If a monostable pulse (duration 0.275)occurs because of an R-wave fed back from the heart or a pacer pulse,this stage 309 resets the multivibrator rate capacitor (not shown) tonear zero volts. Every time the multivibrator 311 is reset it waits for850 milliseconds (70.6 beats per minute) before it puts out anotherpacer pulse. Every time the pacer 21 itself puts out a heart pacingpulse it resets itself.

The noise-rate turn-on 307 constantly monitors the output of theamplifier 303. If the noise rate circuit senses that the amplifiersoutput pusles are at a specific frequency (15 hertz i5 hertz, the noiseturn-one frequency) it disables the monostable stage 305. This in turnprevents the multivibrator reset stage 309 from discharging the ratecapacitor (not shown) in the multivibrator 311. If the rate capacitor inthe multivibrator is not discharged, which happens every time themonostable stage 305 puts out its 0.275 second pulse, the multivibrator311 continues to run, but the capacitor does not discharge to zero onevery pulse and the rate increases to 85:5 beats per minute (the noiserate). When the magnetic reed switch 33 is closed by a magnet, externalof the body, the pacer 21 becomes a fixed rate unit at the noise rate.

In the heart pacer under the control of the circuit shown in FIG. 13R-waves, when they occur, disable the multivibrator 311 from deliveringheart-pacer pulses to the heart. Pulses are only delivered in theabsence of R-waves. The operation of such a heart pacer is referred toas an R-wave-inhibited demand pacer.

Typical parameters of the heart pacer in accordance with this inventionare:

1. Pulse Duration l.l milliseconds, nominal 1.0-1.2 milliseconds, range.A sufficiently long pulse duratino to always insure heart capture isprovided. A minimum pulse duration is desired to conserve electricalpower output. If the pulse duration were reduced substantially below the1.1 milliseconds, the amplitude requirement may become excessive,thereby causing fibrillation.

2. Pulse Amplitude 8 milliamps, nominal 7.5 to

8.5 milliamps, range. This amplitude is selected to always provideeffective capture after the rise of threshold levels after electrodeendothelialization. A higher amplitude, possibly milliamps, may beconsidered, since a nuclear powered system is not battery energysensitive, but the possibility of fibrillation might arise with suchhigh levels. A lower amplitude in some are cases could prevent heartcapture.

3. Basic Rate 70 beats per minute, nominal 68 to 72 beats per minuterange. This rate appears to be the most common required by patients.

4. Noise Rate 85 beats per minute, nominal 80 to 100 beats per minute,range. This is the rate at which the pacemaker operates when noiseinterference is so great that it masks the normal O-R-S heart complex. Arate higher than the basic rate is selected to minimize the possibilityof competition with the normal heart rate should it be in normal rhythm.

5. Noise Rate Turn-On Approximately Hertz. This rate is selected toprevent interference from all conceivable noise modes, microwave ovensand the like, with the exception of a sporadic impulse which would notbe fatal. It is sufficiently low to rule out 60 Hertz rates which arethe most probable, particularly with the expanding use of microwaveovens.

6. Magnetic Switch Rate 85 beats per minute, nominal 80 to 100 beatsminute, range. This rate is a fixed rate, at which the pacer operateswhen energized by a magnet placed near the patients magnetic reedswitch. This turnon feature allows checking of the pacemaker performancesince in the R-wave inhibited mode with normal Q-R-S complexes, thephysican cannot tell whether the pacemaker is operating.

7. R-Wave Sensitivity i 1.75 millivolts, nominal 1.5 to 2.0 millivolts,range. This sensitivity is selected to assure sensing of the R-wave, andis sufficiently high to prevent interference from abnormal P waves.

8. Refractory Period 275 milliseconds, nominal 250 to 300 milliseconds,range. This electronic refractory period begins with either a normalR-wave or the pacemaker stimulus. It is sufficiently long to preventstimuli during the T-wave onset or decaythis period being the criticalperiod for possible pacemaker induced fibrillation, i.e., no stimulusoccurs during the T-wave of normal heart repolarization.

In installing the pacer according to this invention in the body of thehost, the surgeon makes an incision 401 in the body of the host 402preferably in the chest above the heart. The catheter 39 is then passedthrough a vein to the heart muscle. The catheter is then inserted in thepacer 21 and the pacer is inserted in the opening with the insulatingcoating 50 in contact with the muscles 403, FIG. 14, or 404, FIG. 15,and the window or dimple 51 away from the muscles 303 and in engagementwith a non-muscular part of the chest, either the rib cage 405, FIG. 14,or the skin. 406, FIG. 15.

While a preferred embodiment of this invention has been disclosedherein, many modifications thereof are feasible. Thisinvention is not tobe restricted except insofar as is necessitated by the spirit of theprior art.

We claim:

1. A heart pacer for a host including a primary source of heat energy, aconverter connected to said source for converting said energy intoelectrical energy, means, connected to said converter, for derivingelectrical pulses from said electrical energy, outputconductor meansincluding a catheter, connected to said deriving means and to beconnected to the heart of said host, for impressing said pulses on saidheart, and a container for said source, converter and deriving means,the said container being composed of electrically conducting materialand being electrically connected to said deriving means to serve asground therefor, the outer region of said container being coated withelectrically insulating material except over a relatively small limitedarea thereof on one side only, the conducting material of said limitedarea to be connected electrically to non-muscular body parts of saidhost connecting said ground to the body of said host.

2. The heart pacer of claim 1 wherein the limited area is in the form ofa dimple extending outwardly from the remainder of the container.

3. The heart pacer of claim 2 wherein the insulating coating is flushwith the outer surface of the dimple.

4. The heart pacer of claim 1 wherein the deriving means includeselectrical circuit-component means for converting the electrical energyfrom the converter into pulses, and the output circuit-conductor meansincludes output assembly means, said output-assembly means including afeed-through assembly means, said output assembly being connectedthrough the feedthrough assembly means to the deriving means within thecontainer and extending out of the container and being connected outsideof the container to the catheter.

5. The heart pacer of claim I wherein the container is of generallyellipsoidal form and the small limited area is a dimple of generallyoval form.

1. A heart pacer for a host including a primary source of heat energy, aconverter connected to said source for converting said energy intoelectrical energy, means, connected to said converter, for derivingelectrical pulses from said electrical energy, output-conductor meansincluding a catheter, connected to said deriving means and to beconnected to the heart of said host, for impressing said pulses on saidheart, and a container for said source, converter and deriving means,the said container being composed of electrically conducting materialand being electrically connected to said deriving means to serve asground therefor, the outer region of said container being coated withelectrically insulating material except over a relatively small limitedarea thereof on one side only, the conducting material of said limitedarea to be connected electrically to non-muscular body parts of saidhost connecting said ground to the body of said host.
 2. The heart pacerof claim 1 wherein the limited area is in the form of a dimple extendingoutwardly from the remainder of the container.
 3. The heart pacer ofclaim 2 wherein the insulating coating is flush with the outer surfaceof the dimple.
 4. The heart pacer of claim 1 wherein the deriving meansincludes electrical circuit-component means for convertinG theelectrical energy from the converter into pulses, and the outputcircuit-conductor means includes output assembly means, saidoutput-assembly means including a feed-through assembly means, saidoutput assembly being connected through the feed-through assembly meansto the deriving means within the container and extending out of thecontainer and being connected outside of the container to the catheter.5. The heart pacer of claim 1 wherein the container is of generallyellipsoidal form and the small limited area is a dimple of generallyoval form.